1. Field of the Invention
The invention disclosed herein relates to a sensor for measuring at least one of temperature, pressure and force. In particular, the sensor is used with a logging instrument in a borehole.
2. Description of the Related Art
In exploration for hydrocarbons, it is important to make accurate measurements of various properties of geologic formations. In particular, it is important to determine the various properties with a high degree of accuracy so that drilling resources are used efficiently.
Generally, oil and gas are accessed by drilling boreholes into the subsurface of the earth. The boreholes also provide access for taking measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. Well logging can also be used to take measurements of conditions in the boreholes. The conditions in the boreholes are important to know to safely and efficiently use drilling resources.
In one embodiment, a “logging instrument” is lowered on the end of a wireline into a borehole. The logging instrument sends data via the wireline to the surface for recording. Output from the logging instrument comes in various forms and may be referred to as a “log.” Many types of measurements are made to obtain information about the geologic formations and conditions in the borehole. Two important logs are a temperature log and a pressure log.
The temperature log records temperature in the borehole at various depths. The temperature log can provide indication of temperature gradients in the borehole. The temperature log can be compared to a reference temperature log. Departures from the reference temperature log can indicate entry of fluids into the borehole. Conversely, the departures can indicate fluids exiting the borehole. In addition, the temperature log can be used to detect leaks in a borehole casing or leaks from a valve.
The pressure log records pressure at various depths within the borehole. Accurate pressure measurements can be used to monitor depletion of reservoirs associated with the production of hydrocarbons. Further, accurate measurements of pressure in the borehole are needed during drilling operations. It is important to monitor pressure during drilling operations to keep the pressure under control. If the pressure is not kept under control, then an uncontrolled release of oil and gas to the surface (known as a “blowout”) can result. The blowout can cause personal injuries, drilling rig damage, environmental damage, and damage to underground reservoirs.
Another important measurement for analyzing a formation is that of the force of gravity or gravitational acceleration. For example, changes in gravitational acceleration can be related to depletion of hydrocarbons in a reservoir. In addition, absolute gravitational acceleration or changes in gravitational acceleration can be related to the true vertical depth (TVD) of the logging instrument in the borehole.
Measuring the TVD is especially important in a borehole deviated from the vertical. Logging data is generally correlated to the TVD at which the logging data was obtained. It is important for the TVD to be accurate or the logging data can be corrupted. In a deviated borehole, the length of the wireline cannot be relied on to provide an accurate indication of the true vertical depth of the logging instrument because some of the wireline will be in a horizontal orientation. Even in a non-deviated borehole, the wireline can be subject to stretching thereby leading to an inaccurate measurement of the TVD.
In addition to measuring pressure, temperature, and gravitational acceleration, it is also important to be able to measure force such as a force of acceleration or a force imposed on a static object. By measuring force, motions and stresses related to a drill string can be monitored.
Therefore, what are needed are techniques to measure temperature, pressure, and force within a borehole. In particular, the techniques provide for high accuracy measurements.